@jbouw 

Below is the instruction to setup Time Machine on the router. 

https://www.tp-link.com/support/faq/1564/

As you can see, there is no option to set login username and password under Time Machine page; instead, it uses the same U/P for the HDD to access it. 

You can give it a go first; if does not work, please show us two pictures-- first is the picture of the USB storage Device page on AX6000; second is the picture on your MAC. 

Good day. 

@jbouw did you ever resolve this?  I have the same issue... always getting a OSStatus error 2.   No other password besides the TP Link admin has been set...

Same problem

@yavoh000 

Hello, below is the instruction to set up the Time Machine feature on the Archer AX6000, you can confirm whether the configuration steps are correct.
https://www.tp-link.com/support/faq/2729/

Have you ever encrypted your USB disk? What is the model number of it and what about the capacity and formats?

Please show us a screenshot of the error message you saw as well.

BTW, you may check if matt_film's suggestions will help:

https://community.tp-link.com/en/home/forum/topic/187834?replyId=435004

@jbouw  I was having this same issue. What ended up fixing it for me was adding "access authentication" (aka username and password) to the disk in the router admin. See attached #1. When Time Machine asked to connect to the router, I then entered these credentials and it was able to add "BackupTM" as a Time Machine disk. 

Step #2... even though that added the disk to Time Machine, MacOS couldn't actually start a backup. I need to ALSO go into Finder and connect to the disk there. See attached #2. 

After that, Time Machine started backing up fine. We shall see how sticky these settings are. Fingers crossed it will "just work" from now on.

Hope that helps someone.